The ‘Chibanian Age’ Is the First Geologic Period Named After a Site in Japan

The period is named for Japan’s Chiba prefecture, where a cliff shows evidence of the most recent reversal of Earth’s magnetic field

Chibanian Age cliffs
The Chiba cliff section along the Yoro River in the city of Ichihara shows traces of a reversal in the Earth's magnetic field. Photo by The Yomiuri Shimbun via AP Images

Last month, the International Union of Geological Sciences formally adopted the name “Chibanian Age” for the period between 770,000 and 126,000 years ago, Kyodo News reported at the time.

The beginning of the period is defined by the most recent reversal of Earth’s magnetic field, called the Brunhes-Matuyama reversal. The flip took about 22,000 years from start to finish, according to a 2019 paper in Science Advances. Signs of the Brunhes-Matuyama reversal, which was named for pioneers in the research of Earth’s magnetic field, can be found around the world. But a cliff wall in Japan’s Chiba prefecture holds some of the clearest and most extensive evidence for the major geological event.

“In that section in Chiba, you have one of the best records of the reversal interval of anywhere in the world,” geologist and IUGS secretary general Stanley Finney tells Tim Hornyak at Eos. “It’s a significant record of past Earth history that helps us see what may happen now.” Research suggests that another magnetic flip may be due soon.

The Chiba cliffs, located near the city of Ichihara southeast of Tokyo, are home to a sedimentary deposit called the Kazusa Group, which is nearly two miles of rock layers made of compressed silt and clay that were once under the ocean. Volcanic ash in the layers indicates its age is about 770,000 years old. As Erin Blakemore writes for the Washington Post, when the rock was molten, minerals with iron suspended in the molten soup pointed in the direction of Earth’s magnetic field.

When the rock solidified, it trapped the iron-containing minerals in whatever direction they were pointing at the time. Scientists today can look at the captured iron like a time capsule that indicates the history of Earth’s magnetic field. The magnetic field protects Earth’s surface from the radiation of outer space, and as it shifts, that protection becomes weaker.

Because Earth’s geological activity tends to squash older rocks that would hold the captured iron, finding trapped iron is challenging. That’s what makes the Chiba cliffs special: they hold clear signals of changing polarity and an abundance of microscopic marine fossils, as Makoto Okada, a paleomagnetic expert at Ibaraki University, tells Eos.

The team of researchers studying Chiba applied to the IUGS for consideration of the “Chibanian” title in 2017, but the deliberations were delayed when another research team in Japan accused them of misrepresenting their data, Mainichis Yui Shuzo reported at the time. During part of the dispute, the team opposing the Chibanian name obtained the right to lease the land with the research site, and blocked researchers from entering, Yumi Nakayama reported for the Asahi Shimbun.

The clash was settled after the pro-Chibanian research team clarified that although a 2015 study used data from both Chiba and another location, the 2017 application included only data from the Chiba cliffs.

"It's the result of the Japanese team's efforts at meticulously accumulating pieces of evidence," Tohoku University micro-paleontologist Hiroshi Nishi told Kyodo News. "There were twists and turns but IUGS recognized the Japanese team's data as scientifically correct and most appropriate. It's a huge contribution to the progress of Japanese geology."

The Chibanian Age is the first time Japan has been represented in a geological age—most geological timeframes are named for sites in Europe. The decision has been welcomed by communities near the cliffs. Ichihara mayor Joji Koide wrote in a special leaflet to the city that they will prepare for worldwide attention and tourism from the site, per Eos. The site will soon include more extensive facilities in addition to the temporary visitor center currently erected nearby and a “golden spike” marking it as a "global boundary stratotype section and point" will be placed at the site during a dedication ceremony.

“At many of these sites, we have great monuments for illustrative purposes or panels or geoparks,” Finney tells Eos. “These are international geostandards, and you can’t take them into a museum; it’s something there in the field.”

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